1. Field of the Invention
The present invention relates generally to a shift control apparatus for an automatic transmission of a motor vehicle, and more particularly to a technique for preventing a vehicle driver's feeling of an engine racing when the vehicle is re-accelerated after deceleration with the transmission shifted down to the lowest-speed gear position.
2. Discussion of the Prior Art
As an automatic transmission (abbreviated as AT) of a motor vehicle which is selectively placed in one of a plurality of gear positions, there is known an automatic transmission of the type which includes a low-speed gear position that is established by a shifting action including engagement of a one-way clutch, and a high-speed gear position that is established by engagement of a frictional coupling device. For controlling the automatic transmission of this type, there is proposed a shift control apparatus as disclosed in JP-B2-51-18574, which is equipped with shift control means for automatically shifting the automatic transmission according to a predetermined shift control pattern, on the basis of the rotating speed of the engine and a currently required output of the engine, in case that a vehicle speed sensor normally used for controlling the transmission fails to detect the running speed of the vehicle. The predetermined shift control pattern consists of a shift-up boundary line and a shift-down boundary line, which represent relationships between the engine speed and the opening angle of a throttle valve indicative of the currently required output of the engine. These boundary lines are determined such that the shift-up or shift-down engine speed increases as the throttle opening angle increases. In operation, the automatic transmission is shifted down to the next lower-speed gear position if a point indicative of the current engine speed and the current throttle opening is shifted across the shift-down boundary line, and is shifted up to the next higher-speed gear position if the above point is shifted across the shift-up boundary line.
With the known shift control apparatus utilizing the above-described shift control pattern for shifting the transmission on the basis of the throttle opening and the engine speed, the transmission is shifted down to the lower-speed gear position as the engine speed is lowered. For example, the transmission is shifted down to a 2nd-speed gear position established by engagement of a one-way clutch, when the vehicle is decelerated before it is stopped. During the vehicle deceleration with the transmission placed in the 2nd-speed position, the power of drive wheels is not transmitted to the engine through the one-way clutch, and the engine speed is rapidly lowered to its idling speed below a shift-down point on the shift-down boundary line. As a result, the transmission is further shifted down to the 1st-speed gear position. If the vehicle is re-accelerated by depressing the accelerator pedal following the deceleration thereof, the engine will race, that is, the speed of the engine will be increased from the idling speed to a level determined by the current vehicle speed and the currently selected gear position (speed reduction ratio) of the transmission, without the engine power being transmitted from the engine to the drive wheel. During this period, the engine is held in its racing state. This engine racing is unexpected to the vehicle driver.
When the load acting on the engine is increased due to an operation of an air conditioner or other auxiliary equipment, or when the engine is cold, a suitable idling speed control device is activated to raise the idling speed to a level which is higher by a given value than the normal idling speed. Therefore, the shift-down boundary line of the shift control pattern used in this case must be set such that the shift-down engine speed is higher than the raised idling speed. This arrangement permits the transmission to be shifted from the 2nd-speed position to the 1st-speed position at a relatively high shift-down point (engine speed) during deceleration of the vehicle. However, the racing of the engine undesirably continues for a significantly increased time, upon re-acceleration of the vehicle, namely, until the one-way clutch is engaged to transmit the power from the engine to the drive wheels. This prolonged engine racing is unexpected and uncomfortable to the driver.